1. Field of the Invention
The present invention relates to an optical head device, and an optical information apparatus, a computer, a video recording/reproducing apparatus, a video reproducing apparatus, a server, and a car navigation system provided with such an optical head device.
2. Description of the Related Art
Recently, DVD (Digital Versatile Disc) attracts attention as a large-capacity optical recording medium because the DVD can record digital information at a recording density about 6 times as large as that of CD (Compact Disc). As for DVD, there are various standards such as a recordable type and a rewritable type, for example, DVD-RAM, DVD-R, DVD-RW and the like. In DVD-RAM, there are two types, i.e., a type having a track pitch of 1.48 μm and a recording capacity of 2.6 GB, and a type having a track pitch of 1.23 μm and a recording capacity of 4.7 GB. The track pitch of DVD-R and DVD-RW is 0.74 μm. As described above, the track pitches are different depending on the types of DVD.
When the recording and reproduction for a plurality of types of disks with different track pitches are performed by a single optical head device, it is impossible to obtain a tracking error signal in good conditions by a general 3-beam tracking system because of the difference in track pitches. In view of this problem, Patent Document 1 (Japanese Laid-Open Patent Publication No. 2004-145915) proposes an optical head device by which a tracking error signal can be obtained in good conditions from all types of disks having mutually different track pitches.
With reference to the drawings, the optical head device by which a tracking error signal can be obtained in good conditions from any of types of disks having mutually different track pitches will be described.
FIG. 14 is a diagram showing an optical head device 140. The optical head device 140 includes a light source 141, a diffraction grating 142, a half mirror 143, a collimator lens 144, an objective lens 145, a detection lens 147, and a photo detector 148.
The light source 141 includes a semiconductor laser device, and emits coherent light with which a recording layer of an optical recording medium 146 is irradiated in recording/reproducing. The diffraction grating 142 is an optical element for diffracting and separating the light emitted from the light source 141 into at least three light beams. A tracking error signal can be obtained by using the diffracted light beams generated by the optical element. The diffraction grating 142 will be described later in detail.
A multi-layer film is formed in the half mirror 143. The half mirror 143 reflects 50% of the incident light, and transmits 50% thereof. The collimator lens 144 converts divergent light emitted from the light source 141 into parallel light. The objective lens 145 focuses the light onto a recording layer of the optical recording medium 146. An outgoing face of the detection lens 147 is a cylindrical face. For performing detection of a focus error signal by astigmatism, the detection lens 147 applies astigmatism to the incident light. The photo detector 148 receives light reflected from the recording layer of the optical recording medium 146 and converts the light into an electric signal.
Next, the operation of the optical head device 140 will be described in more detail. The light emitted from the light source 141 is diffracted into at least three light beams by the diffraction grating 142. The diffracted light beams are reflected in the direction toward the optical recording medium 146 by the half mirror 143, and converted into parallel light by the collimator lens 144. The parallel light is focused on a recording plane of the optical recording medium 146 by the objective lens 145. The three light beams formed by the diffraction grating 142 are independently focused on the recording plane of the optical recording medium 146, thereby forming three focused light spots. FIG. 15 shows the three focused light spots 149a, 149b, and 149c. Guide grooves 151 are formed at regular intervals on the optical recording medium 146. The focused light spots 149a, 149b, and 149c are formed in a substantially straight line so as to be simultaneously incident on one and the same guide groove 151.
The light reflected from the recording plane of the optical recording medium 146 is transmitted through the objective lens 145, the collimator lens 144, and the half mirror 143. The transmitted light is also transmitted through the detection lens 147 for applying astigmatism to the light. Then, the transmitted light is incident on the photo detector 148. The photo detector 148 performs photoelectric transfer in accordance with the incident light, thereby producing electric signals for obtaining information signals and servo signals (a focus error signal for focus control, and a tracking error signal for tracking control).
The electric signals produced by respective light receiving planes 148a, 148b, and 148c of the photo detector 148 shown in FIG. 15 are input into arithmetic circuits (subtracters 152a, 152b, and 152c, an accumulator 153, an amplifier 154, and a subtracter 155), thereby obtaining a tracking error signal.
The photo detector 148 has two-divided light receiving planes which are divided in a direction corresponding to a direction orthogonal to a radial direction of a disk (that is, a tangential direction of the disk). Based on a difference between signals output from the respective divided light receiving faces, push-pull signals corresponding to the respective focused light spots are detected. Generally, a push-pull signal is detected based on a difference between output signals from two-divided light receiving planes which are divided in a direction corresponding to a radial direction of a disk. However, herein, the astigmatism is adopted for detecting a focus error signal, so that the intensity distribution of the light spot on the light receiving plane is rotated by about 90 degrees around the optical axis. Accordingly, the push-pull signal is detected based on a difference between the output signals from the two-divided light receiving faces divided in the direction corresponding to the tangential direction of the disk.
Next, the diffraction grating 142 will be described in more detail. FIG. 16 is a perspective view showing a lattice pattern of the diffraction grating 142. In a lattice plane of the diffraction grating 142, lattice grooves are formed at regular intervals. The lattice plane is divided into at least three regions by dividing lines orthogonal to the direction in which the grooves are extended. In other words, the lattice plane is divided into at least three regions in a direction corresponding to the tracking direction of the optical recording medium 146. In an example shown in FIG. 16, the lattice plane is divided into three regions 161, 162, and 163 by dividing lines L1 and L2. The center region 162 has a predetermined width W.
With respect to the phase of the lattice grooves in the center region 162, the phase of the lattice grooves in the region 161 adjacent to the region 162 is shifted by +90 degrees. That is, the arrangement of the lattice grooves in the region 161 is shifted by about ¼ of the lattice groove interval with respect to the lattice grooves in the center region 162. On the other hand, the phase of the lattice grooves in the region 163 which is adjacent on the opposite side is shifted by −90 degrees with respect to the lattice grooves in the center region 162. That is, the arrangement of the lattice grooves in the region 163 is shifted by about ½ of the lattice groove interval on the side opposite to the lattice grooves in the region 161. Accordingly, the phase of the lattice grooves in the region 161 and the phase of the lattice grooves in the region 163 are mutually shifted by 180 degrees (i.e. ½ of the lattice groove interval).
The light transmitted through the diffraction grating 142 having the above-described lattice pattern is focused by the objective lens 145, so as to form the light spots 149a, 149b, and 149c on the recording plane of the optical recording medium 146 (see FIG. 15). With respect to the push-pull signal obtained from the light spot 149a corresponding to zero-order diffracted light formed by the diffraction grating 142, the phases of the push-pull signals obtained from the light spots 149b and 149c corresponding to ±1st-order diffracted light formed by the diffraction grating 142 are inverted. All of the light spots are formed on one track, so that there is no problem even if the track pitches are different depending on the types of optical recording mediums. The deterioration degree of the tracking error signal for the lens shift is determined depending on the width W of the center region. As the width W increases, the deterioration degree decreases. However, as the width W increases, the tracking error signal when the lens shift is zero is reduced.
In the optical head device 140, the three light spots are formed on one track, so that the quality of the tracking error signal does not depend on the track pitch. Accordingly, stable tracking error signals can be obtained from various optical recording mediums having mutually different tracking pitches, so that stable reproduction and recording can be performed for the respective optical recording mediums.
However, when a dual-wavelength light source in which two light emitting elements for emitting light with mutually different wavelengths are mounted in one module is used as the light source 141 (the two light emitting elements are disposed along a radial direction), the two light emitting elements are attached in separate positions, so that the centers of light spots of the light emitted from the two light emitting elements formed by the diffraction grating 142 are mutually deviated. For this reason, when the center of the light spot corresponding to one light emitting element is matched with the center of the center region 162, the center of the light spot corresponding to the other light emitting element is deviated from the center of the center region 162. This disadvantageously looks like a condition where the lens shift occurs from the beginning. Accordingly, when the light of which the center of the light spot is matched with the center of the center region 162 is used, stable tracking control can be performed, but when the light of which the center of the light spot is not matched is used, stable tracking control cannot be performed, thereby deteriorating the recording/reproducing properties.